In a drawing process of quartz glass when manufacturing an optical fiber, coating resin is immediately applied to its circumference to prevent a reduction in strength of the optical fiber. UV curing resin is mainly used as coating resin for an optical fiber. Urethane acrylate based resin or epoxy acrylate based resin is used as UV curing resin.
In an optical fiber, transmission loss increases due to various external stress and microbending caused thereby. Therefore, to protect the optical fiber from such external stress, an optical fiber is generally coated in a two-layer structure. Resin having a relatively low Young's modulus is used for an inner layer which contacts the quartz glass as a buffer layer (hereinafter referred to as a “primary layer”) and resin having a relatively high Young's modulus is used for an outer layer as a protective layer (hereinafter referred to as a “secondary layer”). Resin having Young's modulus of 3 MPa or less is generally used for the primary layer and resin having Young's modulus of 500 MPa or more is used for the secondary layer.
An optical fiber is manufactured by coating a quartz glass optical fiber drawn by heating/melting a preform having quartz glass as the main ingredient in a drawing furnace with liquid UV curing resin using a coating die and then irradiating this optical fiber with UV rays, causing the UV curing resin to cure and thereby coating it with the primary and secondary layers. By further coating the circumference of the optical fiber obtained with a coating layer made of colored resin or the like, a colored optical fiber is manufactured. FIG. 1 shows the structure of the colored optical fiber. In the present specification, suppose such a glass optical fiber coated with a primary layer and a secondary layer is referred to as an “optical fiber”, the optical fiber, the circumference of which is further coated with a colored coating layer made of colored resin or the like is referred to as a “colored optical fiber” and a plurality of colored optical fiber arranged in parallel and collectively coated with ribbon resin is referred to as an “optical fiber ribbon.” Furthermore, suppose a glass optical fiber having a coating layer on its circumference, that is, an optical fiber and colored optical fiber is collectively referred to as a “coated optical fiber.”
In order to make a high reliability optical fiber which prevents an increase of transmission loss even when used by being immersed in water, there are various proposals such as to improve adhesion between the primary layer and glass optical fiber.
For example, according to Japanese Patent Application Laid-Open No. H03-29907, a fastening force which occurs at an interface between coating and glass due to a coefficient of contraction of the inner diameter of a protective layer and Young's modulus of the protective layer is restricted and an increase of transmission loss is reduced to obtain good transmission loss.
However, Patent Document 1 only discusses a correlation with an increase of transmission loss at a normal temperature and has given no consideration to further problems such as an increase of transmission loss when used in a wide range of temperature or when immersed in water.
With the remarkable widespread proliferation of optical fibers in recent years, the applicable range of optical fiber cables is considerably expanding. This means environments in which optical fiber cables are used become more and more diversified and long-term reliability required for optical fiber cables is becoming more and more stringent. For this reason, colored optical fiber and optical fiber ribbon are required whose transmission loss does not increase even when used immersed in water.
Under such circumstances, colored optical fiber whose transmission loss hardly increases even when exposed in a water-immersed condition are under study. However, there is a limit to handling the above described problems while achieving a balance of adhesion at the interface between layers and it is a current situation that colored optical fibers are put to practical use by combining a configuration to avoid water from reaching the colored optical fiber by considering a cable structure or the material of a cord or a sheath and a configuration to reduce the amount of water that reaches the colored optical fibers, and the reliability thereof is insufficient.
It is an object of the present invention to provide a colored optical fiber and an optical fiber ribbon whose transmission loss hardly increases even when used immersed in water.
It is another object of the present invention to provide a colored optical fiber evaluation method for measuring an amount of thermal expansion of an optical fiber and judging whether or not the colored optical fiber manufactured using the optical fiber has sufficient resistance to various operating environments.